内蒙古中部早元古代造山事件中麻粒岩相低压变质作用

LOW-PRESSURE METAMORPHISM OF GRANULITE FACIES IN AN EARLY PROTEROZOIC OROGENIC EVENT IN CENTRAL INNER MONGOLIA

内蒙古中部早元古代麻粒岩相低压变质地壳,根据富铝片麻岩中的长石种类和岩石组合,可划分为南北2个带、6个岩石组合。它们代表着不同的变质层状岩系序列及其所经历的变质作用的温度、压力条件和构造背景。矿物的包裹结构和反应边组构,记录了高温低压矿物组合取代了中温高压矿物组合:石榴子石+石英→紫苏辉石+斜长石,蓝晶石→矽线石及石榴子石+蓝晶石/矽线石+石英→堇青石。岩石的变形组构、矿物组合的转变关系和变质作用的PTt轨迹表明:本区与变质作用同期的高温正滑韧性剪切作用,是麻粒岩相低压变质地壳形成的主要原因。由中温高压变质环境转入高温低压变质环境是造山事件中推覆作用与拉伸作用联合作用的结果。

According to the kinds of feldspar in the Al-rich gneisses and the characteristics of rock associations, the low-pressure Early Proterozoic metamorphic crust in central Inner Mongolia can be divided into the south belt and north bel which are composed of six rock associations. They represent the relevant rock groups of the layered metamorphic rock series, formed under specific metamorphic temperature and prosure conditions as well as tectonic environments. Mineral inclusions and reaction textures record that the medium-temperature high-pressure mineral assemblages are replaced by the high-temperature low-pressure mineral assemblages: garnet+quartz→hyperthene+plagioclase, kyanite→sillimanite and garnet+kyanitel sillimanite+quartz→cordierite. The decreasing pressure condition is indicated by the break-down of garnet in the metapellite and metamafic rocks. Through the studies of geological setting, deformation fabric, variation of mineral assemblages and metamorphic PTt path, the following conclusions can be made. The early Proterozoic metamorphic crust was produced by intraplate collision at about 2100-1800 Ma. It is indicated by the earlier increasing pressure and later decreasing pressure records in studied samples that the crust is thickened and then thinned. The low-pressure metamorphic crust of granulite facies was formed by extension during the uplifting of metamorphic crust. The changes from medium--temperature/high-pressure to high-temperature/low-pressure conditions are the results of thrusting and shearing and crustal extension in an early Proterozoic orogenic event.